1. Field of the Invention
The present invention relates to a driving device for display panels used in AV (audiovisual) equipment, OA (office automation) equipment, computer terminals with a communication function and the like.
2. Description of the Related Art
The desire for a large-scale display apparatus with a large display capacity has recently increased as society has become more information-oriented. In order to satisfy such a desire, a CRT (cathode-ray tube), which is considered to be the best display device in service today, has been developed to be more refined and have a large-scale. For example, a direct view type CRT has attained a size of approximately 40 inches, and a projection type CRT has attained a size of approximately 200 inches. In realizing a large-scale CRT with a large display capacity, however, problems of weight and depth become more severe. Therefore, there is a strong demand for a method for attaining a large-scale CRT with a large capacity without causing such problems.
A flat type display apparatus, which employs a different theory of display from that of the CRT, has been used in word processors, personal computers or the like. A development has been also made in such a flat type display apparatus so as to attain a sufficiently high display quality to be used for an HDTV or a high performance EWS (engineering work station).
The flat type display apparatus is classified into an ELP (electroluminescent panel), a PDP (plasma display panel), a VFD (vacuum fluorescent display), an ECD (electro chlomic display), an LCD (liquid crystal display) and the like. The LCD is regarded as the most promising and has been developed most significantly among those mentioned because it can easily achieve a multicolor display and can be matched with an LSI (large scale integrated circuit).
The LCD is classified into a simple matrix driving type LCD and an active matrix driving type LCD. The simple matrix driving type LCD has a structure in which liquid crystal is enclosed in an XY matrix type panel comprising a pair of glass substrates respectively bearing electrodes in the shape of stripes formed thereon. The glass substrates are opposed to each other so as to make the electrodes on one of the substrates vertical to the electrodes on the other substrate. This type of LCD utilizes sharpness of liquid crystal display characteristics to display an image. The active matrix driving type LCD has a structure in which nonlinear elements are directly connected to pixels, and positively utilizes nonlinear characteristics such as a switching characteristic of each element for displaying an image. Therefore, the active matrix driving type LCD depends upon the display characteristics of the liquid crystal itself less than the simple matrix driving type LCD, and can realize a display with high contrast and fast response. The nonlinear elements used in the active matrix driving type LCD are divided into two types: a two-terminal type and a three-terminal type. Examples of the two-terminal type nonlinear element include an MIM (metal-insulator-metal), a diode and the like. Examples of the three-terminal type nonlinear element include a TFT (thin film transistor), an Si-MOS (silicon metal oxide semiconductor), SOS (silicon on sapphire) and the like.
In spite of the above-mentioned advantages of the active matrix driving type LCD, the simple matrix driving type LCD is advantageous in the production cost because it has a simpler display panel structure.
In the simple matrix driving type LCD, the ratio of the effective voltage applied to a selected pixel to that applied to a non-selected pixel becomes almost 1:1 as the number of scanning electrodes increases. Therefore, in order to attain high contrast, the liquid crystal used in such an LCD is required to have sharpness of the display characteristics. An STN (super twisted nematic) LCD is generally used for achieving this sharpness. In the STN LCD, the liquid crystal molecules are twisted through an angle of approximately 180.degree. to 270.degree., and a polarizer is further used. In addition, an STN LCD further including a compensator made from liquid crystal or a polymer film is commercially available.
The response characteristic of an LCD is generally contradictory to the contrast characteristic thereof. This can be partly explained by the driving voltage waveform of the LCD. In the XY matrix driving method usually used in the simple matrix driving type LCD, each of the scanning electrodes is successively selected, and synchronously with the selection, signals corresponding to display data are applied to data electrodes vertical to the scanning electrodes at a time. In this method, the voltage applied to each pixel can be indicated as FIG. 8A. During one frame while all the scanning electrodes are successively selected to be turned on, a high voltage T is applied at least once, otherwise, a constant low bias voltage U is mainly applied.
In a fast responding LCD, which is realized by using a liquid crystal material having optimal characteristic values such as viscosity and layer thickness, the transmission of the LCD varies, as shown in FIG. 8B, in response to the above-mentioned variations between the voltages T and U. Such phenomena will be hereinafter referred to as the "frame response phenomena". Because of the phenomena, the transmission deviates from an optimal effective response line of the applied voltage, which is shown with a dashed line in FIG. 8B. As a result, the contrast of the LCD is degraded.
The following two methods have been recently proposed as a driving method for suppressing the frame response phenomena: One is the so-called active addressing system. In this method, while positive or negative voltages derived from the Walsh function are simultaneously applied to all the scanning electrodes, data signals correlated with display data input from the outside are transferred to the data electrodes synchronously with the application of the voltages (T. J. Scheffer, et al., SID '92, Digest, p. 228). The other is the so-called multiple line selection system. In this method, positive or negative voltages based on the binary system or voltages of 0 are applied to a plurality of scanning electrodes (T. N. Ruckmongathan, 1988 IDRC p. 80).
An example of the specific procedure in the active addressing system will now be described. Scanning signals Y.sub.n (n=1 to 5) for a dot matrix of five columns by five rows as shown in FIG. 9 are determined by using the Walsh function as shown in FIGS. 10A and 10B. Specifically, five different kinds of signal patterns are applied to the respective scanning signals Y.sub.n as shown in FIG. 10A. One frame is divided into eight terms t.sub.1 to t.sub.8. The on state is taken as +1 and the off state is taken as -1. Under these conditions, the signal patterns of the scanning signals Y.sub.n in one frame are shown with +1 and -1 as in FIG. 10B.
Next, data signals X.sub.m (m=1 to 5) are obtained as follows: FIG. 11 shows the data signal when m=2. Display data I.sub.km (k=1 to 5) for the respective dots in the mth column are indicated with one of the two values: -1 (the on state) and +1 (the off state). The value of the display data I.sub.km is multiplied by the scanning signal Y.sub.k. FIG. 13A shows Y.sub.k I.sub.km, the results of the multiplication in the case of m=2. Then, the obtained results are added with k in each term, thereby obtaining added values g.sub.m as shown in FIG. 12A. In FIG. 12B, the added values g.sub.m are indicated as a voltage level when m=2.
The data signal X.sub.m is indicated as a product obtained by multiplying the added value g.sub.m by a constant C. The constant C depends upon the number N of the scanning electrodes alone, and is represented by an equation described below. When the number N is 5, the constant C is 0.425. ##EQU1##
When all the scanning signals Y.sub.n (n=1 to 5) and the data signals X.sub.m (m=1 to 5) are simultaneously applied to the respective scanning electrodes and data electrodes for a face scanning, the display data I.sub.nm is displayed on the display panel. The arithmetical procedure is as follows: The signal to be applied to each display dot (n,m) is represented by a difference between the signals Y.sub.n and X.sub.m. By conducting the face scanning, an image corresponding to the effective voltage value in one frame is displayed by each display dot. Therefore, the voltage applied to the display dot (n,m) is represented by the following equation: ##EQU2## wherein t.sub.j is a term into which a frame is divided; and I/T is a normalization constant. In the above description, since one frame is divided into eight terms, t.sub.j corresponds to t.sub.1 to t.sub.8, and T is 8. Y.sub.n (t.sub.j) and X.sub.m (t.sub.j) are values of X.sub.n and Y.sub.m in each term t.sub.j, respectively (see FIG. 10). In addition, since Y.sub.n (t.sub.j) is an orthogonal function, the following equations hold: ##EQU3## In this manner, each of the signals is applied to the display dot (n,m) during one frame, and the display data is reproduced on the display dot (n,m).
In FIG. 13A, the display dots in the on state are shown with .circle-solid. and the display dots in the off state are shown with .largecircle.. FIG. 13B shows the voltage waveform of an on-state dot in the second column and the third row and that of an off-state dot in the second column and the fourth row in FIG. 13A.
Next, an example of the specific procedure in the multiple line selection system will be described. For example, a group of three scanning electrodes as shown in FIG. 14 is simultaneously selected, and a voltage of +Vr or -Vr is successively applied to each group for scanning. Therefore, voltages of three values, i.e., +Vr, -Vr, and 0 at the time of non-selection, are used as the scanning voltages in this system.
The display pattern of the on state is taken as 1, and that of the off state is taken as 0. The voltage +Vr of the scanning electrode is taken as 1, and the voltage -Vr is taken as 0. These values are respectively applied to bits, and the exclusive OR operation is conducted to determine the voltage of one data electrode. At this point, the data voltage is required to have M+1 voltage levels if a multicolor display is desired, wherein M is the number of the selected lines, i.e., 3 in the above case.
Next, the scanning voltage and the data voltage determined as above are simultaneously applied to the first group of the scanning electrodes. A similar procedure is repeated with regard to each group of the plurality of scanning electrodes. As a result, the panel displays an image corresponding to the display data.
As is known from the above description, a plurality of selections for the scanning electrodes are performed in one frame in these systems. Therefore, each of the applied voltage values of the respective waves in one frame approaches the average thereof, thereby suppressing the frame response phenomena, which is caused in the conventional method in which only one selection is performed in one frame.
FIG. 15 shows, as an example of the specific circuit, an LCD system having a driving device of an active addressing system. The LCD system has an XY matrix type LCD 1. The LCD 1 comprises a liquid crystal layer, and scanning electrodes 1a and data electrodes 1b oppose each other so as to sandwich the liquid crystal layer therebetween. For example, the data electrodes 1b are 15 electrodes to which data signals X.sub.1 to X.sub.15 are respectively input. The scanning electrodes 1a are 15 electrodes to which scanning signals Y.sub.1 to Y.sub.15 are respectively input. A portion on which each scanning electrode 1a and each data electrode 1b cross each other works as a display dot (a pixel).
The data electrodes 1b are connected to a data electrode driving circuit 4, and the scanning electrodes 1a are connected to a scanning electrode driving circuit 5. The scanning electrode driving circuit 5 has, in each output system, a transfer gate 5a to which a voltage of +Vr is applied and a transfer gate 5b to which a voltage of -Vr is applied, as shown in FIG. 16. The scanning electrode driving circuit 5 selects one of the voltage levels, +Vr or -Vr, on the basis of a timing signal as shown in FIG. 15 to output the scanning signals Y.sub.1 to Y.sub.15 to the respective scanning electrodes 1a.
The data electrode driving circuit 4 has, in each output system, a sampling gate 4a, a transfer gate 4b, a sampling capacitor 4c, a transfer capacitor 4d and an output buffer 4e as shown in FIG. 17. The data electrode driving circuit 4 successively samples the data signals X.sub.1 to X.sub.15, obtained as the results of the calculation, in accordance with the timing signal. When it finishes sampling all the data signals for one scanning electrode, it outputs the sampled data signals to the respective data electrodes 1b.
The data electrode driving circuit 4 receives an output signal from an orthogonal transformation arithmetic circuit 3. The orthogonal transformation arithmetic circuit 3 receives an image data signal, a timing signal and a signal Y that is output by a Walsh function generator 2. The Walsh function generator 2 receives a timing signal. The scanning electrode driving circuit 5 receives a timing signal and a signal Y that is output by the Walsh function generator 2.
In the driving circuit of the active addressing system having the above-mentioned structure, signals are processed as follows: The Walsh function generator 2 provides a signal Y with a voltage waveform indicating the Walsh function. The signal is sent to each of the scanning electrodes 1a through the scanning electrode driving circuit 5. The orthogonal transformation arithmetic circuit 3 divides the image data signals input from the outside into two types of signals, +1 and -1, multiplies each of the signals by the signal Y sent from the Walsh function generator 2, and obtains the respective added values g as described above, thereby obtaining signals X by multiplying the added values g by the constant C. The signals X are sent to the respective data electrodes 1b through the data electrode driving circuit 4. In this manner, when the voltage application for one frame is finished, an original image is reproduced on the LCD 1.
FIGS. 18A, 18B, 18C and 18D respectively show the voltage waveforms of data signal X.sub.1, scanning signals Y.sub.1, Y.sub.7 and Y.sub.15 generated in one frame in the driving circuit of the above-mentioned active addressing system. FIGS. 18E, 18F and 18G show the voltage waveforms in one frame at the display dots to which signals Y.sub.1 to X.sub.1, Y.sub.7 to X.sub.1 and Y.sub.15 to X.sub.1 are applied, respectively. In these figures, the ordinate indicates a voltage value and the abscissa indicates time. +Vr and -Vr are the output voltage values of the scanning electrode driving circuit 5 and Vc(t) is the output voltage value of the data electrode driving circuit 4. In these figures, all the values are calculated under a condition where all the image data are to be displayed in the on state.
FIGS. 19A through 19G show the voltage waveforms when the data signal X.sub.1 has a different voltage waveform from that shown in FIG. 18A.
As is known from FIGS. 18A through 18G, even when all the image data are to be displayed in the same on state, the voltage waveforms at the display dots are significantly different from one another in the driving voltage waveforms and the frequency components depending upon the scanning signals to be applied to the scanning electrodes. Specifically, the waveform shown in FIG. 18E has more low frequency components as compared with the waveform in FIG. 18F, and the waveform in FIG. 18G has further less low frequency components, while the high frequency components increase in this order. This also applies to the waveforms shown in FIGS. 19A through 19G.
Therefore, even when all the image data are to be displayed in the same state, the effective voltage value varies in each display dot due to the difference in the frequency components, resulting in a nonuniform display. The reason is as follows: In an LCD, a low pass filter is formed by resistance components such as an electrode resistance and capacity components in the liquid crystal layer. The frequency components of a voltage applied to each display dot vary due to the low pass filter, resulting in nonuniform effective voltage value. Another possible reason is frequency dependence caused by the characteristics of the liquid crystal material and/or the orientation film in the LCD. Similar problems are caused in the multiple line selection system. Therefore, in either system, display irregularities such as crosstalk are caused, and the display quality is significantly degraded.
The Walsh function will now be described in more detail. When the number L of data is taken as 2.sup.5, a complete one-dimentional Walsh function system with a cycle of L includes L signals Wal(m,n), wherein m=0, 1, 2, . . . , L-1; and n=0, 1, 2, . . . , L-1. For example, when L=2.sup.8, i.e., 256, the Walsh function system includes 256 signals Wal(m,n). Wal(m,n) is defined by the following equations: ##EQU4## In the above equations, [] indicates a Gaussian sign, and [a] indicates obtaining a largest integer equal to or smaller than a.
However, since the number N of the scanning electrodes is optionally setted in an LCD, the number N is generally not equal to the number L (i.e., 2.sup.r). Therefore, in such a case, N signals Wal(m,n) are selected among the 2.sup.5 signals, and a voltage is applied to them. Since the selected Walsh function system is not complete in this case, problems of contrast degradation and the crosstalk are caused. Therefore, it is impossible to perfectly reproduce a desired display image in the conventional LCD.
In addition, since a fixed voltage signal derived from the Walsh function is applied to the fixed scanning electrodes, the voltage waveforms at respective scanning electrodes are different from one another in frequency components. Such a difference is revealed as a difference in the applied voltage due to the capacity of the liquid crystal display panel and wiring resistance in the LCD, thereby also causing crosstalk.